GEODETICAL TABLES. Ixxvii 



Example : 



To find the duration of sunshine, May i8, 1892, in latitude 49° 30' 



North. 

 From the Nautical Almanac, 8=19° 43' A^.^ at Greenwich apparent noon. 

 From the table, with 8=19° 43' N. and ^ = 49° 30', the duration of 

 sunshine is found to be 15'' 31'". 

 Table 95. Declination of the sun for tlie year 1899, at Greemvich apparent 

 noon. 

 This table is an auxiliary to Table 94, and gives the declination of the 

 sun for every third day of the year 1899. These declinations may be used 

 as approximate values for the corresponding dates of other years when the 

 exact declination cannot readily be obtained. Thus, in the preceding ex- 

 ample, the declination for May 18. 1892, may be taken as approximately 

 the same as that for the same date in 1899, viz. 19° 34', 



THE DURATION OF TWILIGHT. 



A review of the literature ^ indicates that from an early date astronomi- 

 cal twilight has been considered to end in the evening and begin in the morn- 

 ing when the true position of the sun's center is 18° below the horizon. At 

 this time stars of the sixth magnitude are visible near the zenith, and gen- 

 erally there is no trace on the horizon of the twilight glow. 



It also appears that civil twilight ends in the evening and begins in the 

 morning when the true position of the sun's center is 6° below the horizon. 

 At this time stars and planets of the first magnitude are just visible. In the 

 evening the first purple light has just disappeared, and darkness compels 

 the suspension of outdoor work unless artificial lighting is provided. In 

 the morning the first purple light is beginning to be visible, and the illu- 

 mination is sufficient for the resumption of outdoor occupations. 



Some confusion has arisen in the computation of tables of the duration 

 of both astronomical and civil twilight, due to the fact that in some in- 

 stances the time of sunrise or sunset has been considered to be that instant 

 when the center of the sun is on the true horizon; in others, when its center 

 appears to be on the true horizon ; and in still others when the upper limb 

 of the sun appears to coincide with the true horizon. In the United States 

 this latter is regarded as defining the time of sunrise and sunset. 



In the tables here presented the duration of astronomical twilight is 

 the interval between sunrise or sunset, according to this latter definition, 

 and the instant the true position of the sun's center is 18° below the horizon. 

 Likewise, the duration of civil twilight is the interval from sunrise or sun- 

 set to the instant the true position of the sun's center is 6° below the hori- 

 zon. 



^ Kimball, Herbert H. " Duration and Intensity of Twilight," Monthly Weather Review 

 1916, 44: 614-620. 



